Cylinder boring vs a lathe
by Richard Sheckler
Why would one use a lathe in preference over a boring device made especially for boring cylinders? That Duane is a fine question, and I delight in your asking because I am about to tell you 😉
A precision lathe like a Clausing can be set up for repeat accuracy using a shop made jig to hold each cylinder, aligning each according to the cylinder bases and skirts. Before boring, the top of each cylinder can and should be checked for concentricity and alignment, one key objective in the use of a lathe. Another is guaranteeing that the center line of the finished bore is perpendicular to the CL of the crankshaft. The third is to torque the cylinder base against the jig the same as would be the finished cylinder against the crankcase, to apply the same stress that occurs when the cylinder is in operation. This ensures the bore will be concentric when attached and torqued to the case.
I have received cylinders returned from prominent boring and cylinder services that had bores as much as .045″ off center and 8 degrees away from perpendicular. Considering that these crankcases spread apart in the front over so many years, when the bore leans to the rear on such a case, the effect is cumulative, and a wrist pin failure is imminent. Worked on too many of these failures, and every time the bores were off the 90-degree mark.
Solution: Deck the crankcase (flycut the mating surfaces for the cylinder bases on the case, equally both sides) and bore the cylinders on a lathe.
My thanks to Richard for this great explanation. This is one more example of how this site for BMW motorcycles got built by many contributors.